The present invention relates generally to electromagnetic imaging and, more specifically, to a system for imaging subterranean leakage from pipes, storage tanks and the like using continuous-wave (CW) signals.
Electromagnetic imaging systems have been used to locate subterranean objects such as land mines, buried pipes, tanks, and archaeological structures. In conventional electromagnetic imaging systems, an antenna is translated over the ground surface, and data are collected at multiple discrete points along the path that the antenna traverses. At each point, a broadband radar produces a series of short duration pulses that contain a range of frequencies. The system achieves resolution in the depth dimension by measuring the travel time of the pulses. Object structure in the horizontal dimensions is obtained from variations in the magnitude of the reflection from the buried object. The system could Fourier transform to the collected data to produce a holographic or three-dimensional image of the object provided phase data were measured; however, conventional pulsed systems do not measure phase, so images are not formed.
Pulsed radar systems are useful for detecting large underground objects such as land mines and archaeological structures. Although the presence and depth of such objects is relatively unpredictable, pulsed systems can image a sufficiently large volume of earth that any objects within it are likely to be detected. However, the image quality of pulsed systems suffers because the broadband pulses are degraded by the dispersion effects of the soil. Furthermore, images are affected if the levels of moisture or impurities in the soil vary over the area imaged because these levels affect pulse attenuation.
In Yue et al., Two Reconstruction Methods for Microwave Imaging of Buried Objects, IEEE Transactions on Computers, vol. c-24, No. 4, pp. 381-390, April 1975, a co-inventor of the present invention discloses a CW subterranean imaging system. The system has an antenna that is translated over the ground by a motor-driven carriage to project a single-frequency wave downward into the ground for a short duration at each point along the path that the antenna traverses. The system measures the amplitude and phase of the reflected wave at each point. A computer executes a Fourier transform that transforms this reflectance data to the spatial frequency domain.
In the above-described system, gathering a sufficient amount of data to form an image is extremely slow because a single antenna is mechanically translated over the ground surface in two dimensions. A manuscript, Davis et al., Nearfield Imaging of Dielectric Anomalies: Antennas and Processing, presented by the co-inventors of the present invention at a conference of the International Commission for Optics at Garmisch, Germany in April, 1990, disclosed an antenna array comprising two rows of identical antenna elements. The elements of the leading row are staggered with respect to those of the trailing row. After the leading row gathers reflectance data for the point over which each element is located, the array is advanced, and the trailing row gathers reflectance data for the points between the points for which the leading row previously gathered data. Thus, the antenna array effectively doubles the number of points in the dimension perpendicular to the direction of travel of the array. Increased sampling reduces errors in subsequent data processing.
Underground pipes and storage tanks are commonly used to transport or store many types of substances, including hazardous waste, petroleum products, natural gas, sewage, and water. Structural defects, corrosion, and seismic activity can rupture underground structures, causing the contents to leak into the earth. Leakage of some of these substances can be extremely injurious to the environment, polluting groundwater and endangering living organisms. The monetary cost for cleansing the earth of such substances can be astronomical, and damage to living organisms can often never be undone. Moreover, leakage of even the most benign substances wastes the valuable resources of both the substance itself and the manpower and materials expended in finding and repairing the leak. Conventional methods for locating such leakage are time-consuming and uneconomical. Every minute that a leak continues undetected compounds the damage and cost.
A fast, economical, and convenient method for locating leakage of underground structures is needed. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.